For example, JP2010-26483A (US2010/0014054A1) teaches a display apparatus, which uses a laser light (a coherent light) to project an image onto a screen member. In this display apparatus, two laser lights, which have orthogonal polarization directions, respectively, that are perpendicular to each other, are generated, and these laser lights are projected onto the screen member to produce the image. Since the polarization directions of these two laser lights are perpendicular to each other, it is possible to limit generation of a speckle noise, which is otherwise generated by interference between diffused lights that are diffused by the screen member upon impingement of the laser lights onto the screen member.
This technique is based on that the lights, which have different polarization directions, respectively, do not substantially interfere with each other. When the lights, which have the different polarization directions, respectively, are projected onto the screen member, the speckle noise is reduced. Here, it should be noted that the viewer recognizes the reduction of the speckle noise when both of the two diffused lights, which are diffused on the screen member upon impingement of the two laser lights having the orthogonal polarization directions onto the screen member, are reflected by the screen member to the eye of the viewer.
As discussed above, the viewer can recognize the reduction of the speckle noise since both of the two diffused lights, which are diffused by the screen member upon impingement of the lights having the different polarization directions onto the screen member, are reflected to the eye of the viewer. When a difference between intensities of the diffused lights is large, the effect of reducing the speckle noise may possibly be deteriorated.
When the technique of JP2010-26483A (US2010/0014054A1) is applied to a head-up display apparatus of a vehicle that projects a display image, which is formed on a screen member, onto a display member (e.g., a windshield of the vehicle) to enable visual recognition of a virtual image of the display image by a viewer (driver), the above effect of reducing the speckle noise generated on the screen member may possibly be deteriorated.
The deterioration of the effect of reducing the speckle noise tends to occur when a polarization direction of one of two orthogonally polarized lights is parallel to a plane of incidence on the windshield. In general, when a polarization direction of one light component (i.e., a component of an electric field) is parallel to a plane of incidence, such a light component is referred to as a p-polarized light component. Furthermore, another light component, which is perpendicular to the plane of incidence, is referred to as an s-polarized light component. A reflectance of the p-polarized light component is lower than that of the s-polarized light component. Therefore, the diffused light, which is polarized in the polarization direction of the p-polarized light component, substantially does not reach the eye of the viewer. Thus, even when the laser lights, which have orthogonal polarization directions, respectively, that are perpendicular to each other, are incident on the screen member to reduce the speckle noise, the intensity of the one of the polarized lights, which are diffused and reflected by the screen member and reach the eye of the viewer, may possibly be substantially reduced in comparison to the intensity of the other one of the polarized lights. Thereby, the effect of reducing the speckle noise may possibly be deteriorated. Particularly, when the angle of incidence of the polarized laser light on the windshield is equal to or close to the Brewster's angle, the intensity of the polarized laser light, which has the polarization direction that is equal to the polarization direction of the p-polarized light component (i.e., that is parallel to the plane of incidence of the polarized laser light), may possibly become very small, and thereby the effect of reducing the speckle noise may possibly be largely deteriorated.